4 NEW ZEALAND TIMBER DESIGN JOURNAL VOL 21 · ISSUE 2

OLD GROWTH: OUR TIMBER ENGINEERING HERITAGE

Hank Bier, Technical Editor This issue's Old Growth comes from Trada Bulletin Vol 3 No. 8 of November 1970 and since the photo shows the

completed structure, it would have been designed around the time that Trus-Joist Corporation in USA was born, with the

development of tubular metal webbed trusses. It is nice to see the New Zealand was also at the edge of these

developments.

* Published in Timber Development Association Bulletin Vol. 3, No. 8, 1970.

COMPOSITE STEEL AND TIMBER SPACE TRUSS*

A wedding of steel and timber occurs every now and then, though surprisingly many people have the idea that the two

are mutually antagonistic. It just isn't so - but timber can do a few things that steel can't, and vice versa. Let's leave it at

that.

Here is something new — a composite timber and tubular steel truss designed in the office of the chief structural

engineer of the Ministry of Works (Mr O.A. Glogau) for the gymnasium of the Paremoremo Maximum Security Prison.

"A truss of this type is of more pleasing appearance than a timber truss with lapping members at the joints such as are

normally required for a truss of 80' span, only 5' high and spaced at 1,3' 4" centres. In any case a timber space truss

cannot be formed by conventional means. Subject to the availability of suitable timbers a composite truss uses less

imported material than a full steel truss," writes Mr Glogau.

"Top and bottom chords for the truss described were

two 8" x 4" dressed timbers supporting an ex 6" x 2"

dressed diagonally sarked roof. Web members were 2" x

2" x 0.192 R.H.S. The longest available lengths of timber

should be used for the chords to avoid the very much

larger plates required in positions where they must be

spliced. The triangular shape results in only half the

number of bottom chords being needed for a given

spacing of top chords.

"The thickness and available length of timbers for

diagonal sarking influences the maximum spacing of the

top chords. In diaphragms staggered lapse of sarking

are required but this continuity of sarking also reduces

its deflection and thus allows wider spacing of

supporting members. The sidewards leaning web

members stabilise the bottom chords against

compression buckling under reversing loads such as

can be caused by wind suction on relatively light roofs.

"No particular difficulty was experienced during the

construction but the specification should make it clear

which sub-contractor (timber or steel) will make his

shop floor available for assembly of the composite

trusses.

"The trusses were precambered 8½". The design of the

joints must involve careful examination of the load path

through the various splice plates and connections to

avoid or allow for bending stresses, particularly in the

horizontal portion of the splice plates. Similarly the

balance of forces in the joints of the top chord, including

those during the erection stage prior to placing of the

sarking, is an important consideration with this type of

truss."

Figure 1. An isometric of the end of a truss.

Figure 2. The completed structure.